Broadband infrared sum frequency generation vibrational spectroscopy enhanced by post-dual optical parametric amplification

Sum frequency generation (SFG) vibrational spectroscopy is an important tool for probing interfacial molecular structures and dynamics, but its application to weak signals is hindered by limited sensitivity. While post-optical parametric amplification (post-OPA) has enhanced SFG signal intensity, the requirement for separate reference and sample measurements restricts its utility. Herein, we develop a post-dual OPA-enhanced SFG technique integrated with internal heterodyne-detected SFG (HD-SFG) to address these limitations. Using z-cut β-barium borate (BBO) crystal as the substrate—featuring a larger second-order nonlinear susceptibility than z-cut α-quartz—we exploit the orthogonal polarizations of chiral and achiral terms from the BBO surface to split the SFG signal into two paths, which are simultaneously amplified in the same nonlinear crystal and detected by a single CCD. Temporal delay control between the seed SFG signals and the post-OPA pump light optimizes amplification gains for non-resonant and resonant SFG signals, suppressing overwhelming non-resonant backgrounds while boosting weak resonant signals. Experimental validation with methoxy adsorbates demonstrates that the post-dual OPA method significantly improves the signal-to-noise ratio (SNR) and spectral contrast. This approach not only resolves the normalization bottleneck of conventional post-OPA SFG, but also enables us to expand the applicability of SFG spectroscopy to low-concentration species and ultrafast interfacial reactions.